Let's say two sites, A and B host similar content.

The end user cannot see some content on site A because of adblock detection but he can see it on site B, he down votes the link of site A on sharing networks, he asks for a policy banning or advocates against linking to site A because content can be found on B.

I've seen this play out many times, and at that point you're not just losing traffic from ad block clients you're losing traffic overall, have people actively promote your competitors and earn a bad rep (and that rep might not even be objective - in general if people remember anything it will be "don't use A" not the "why" of it)

Perhaps we should add [adwall] to links similar to the way we include [paywall]?

This won't work once more and more sites join the adwall band.

Cartel members always have incentives to cheat - if one site can become popular by not ad-walling like 90% of them do and regurgitate the content of 90% they will do it and disregard "the big picture" especially considering the low barriers to entry. They have no enforcement mechanism to keep everyone honest - the only way this sort of thing works is if you "outlaw" non-ad-walled sites - something like google not listing those sites - otherwise not happening.

A restaurant cannot beat other restaurants by making its meal free to all.

No, but it can by offering better service, better food, better decor etc.

But customer needs to pay

Someone needs to pay in a society where a restaurant owner needs money to eat. But that someone doesn't have to be the customer, doesn't have to be every customer - and the economy might not be structured to require anyone to pay for things, in which case both the restaurant and the customers get everything without money. (Why would people work there? Maybe vocation or hospitality, maybe they wouldn't and robots would instead.)

Oh, good dreams...

I would say just the reverse, this will work more and more.

Under what scenario?

A site which eventually has no visitors will become irrelevant. Those with ad blockers will also often share links to said site which others without ad blockers will click on. If you block me on your website there are many others who will no longer be clicking through via the links I would otherwise have shared.

>You stopped leeching their bandwidth without any opportunity for them to earn any ad revenue from them!

I stopped adding comments, starting discussions, I allow some non-intrusive ads when I enable them, I have quite extensive whitelist in ublock.

Well as a user I might share a link to a site that will be seen by 100 or 1000's of others, on Facebook, Reddit or whatever social network. If that website blocks me from visiting them at first place, they will also loose all the potential revenue from the viral effect.

Is it what you call a leech ?

But your friends have larger chance also being ad blocker users, right? In the end, the website just get more requests and a bigger bill from ISP without getting a cent from those.

You're assuming the set of people who dislike ads and actively block them, intersects with the set of people who don't mind ads.

I'm not convinced that's the case. They're usually quite different demographics.

Is bandwidth anything remotely worth "protecting" in this day and age?